It was the first Saturday in May, and 19 horses were settled into the starting gate at the muddy mess that is Churchill Downs’ main track for the 139th Kentucky Derby.

Rosie Napravnik, who earlier returned from a race with a mud mask that wasn’t courtesy of any day spa, nestled aboard her colt, Mylute. As Napravnik looked down the line, she heard nothing but male voices, saw nothing but male faces. Once again, she was the only female rider in a race, a rare woman in a sport that has long been dominated by men.

Women may make up 49 percent of the work force nationally, but in horse racing, approximately 10 percent of the riders are female. There are more women on the backstretch than ever, and more women playing important roles as owners and trainers in racing. But although the number of female jockeys has grown, only about 75 out of the 750 licensed jockeys are women, according to the Jockey’s Guild. And very few of them get a chance to ride in high-profile races like the Triple Crown series or Breeders’ Cup championships.

Considering that just as many women, if not more, ride horses and show horses from an early age, it raises the question: Why aren’t there more female jockeys?

Those who should know — the sport’s early female pioneers and today’s female riders — say those numbers are more a result of how tough horse racing is than they are sexism or male chauvinism by owners and trainers.

They all agree that there’s a lot of room for improvement in terms of the number of female riders.

One of them is Napravnik. Despite being outnumbered, she and many of her fellow female riders give no quarter to their male rivals. In fact, Napravnik is inching up on racing history, a furlong at a time, a place at a time.

After finishing ninth in the 2012 Kentucky Derby, Napravnik went on to place fifth this year, only slightly behind the third- and fourth-place finishers. It was the best finish for a female jockey in the Run for the Roses.

“We moved up four places from last year, so if we could do that again, we’ll get the win,” Napravnik said.

Two weeks after the Derby, Napravnik became just the third woman to ride in the Preakness Stakes, finishing third aboard Mylute. It was the best finish for a woman in that race. And at this year’s Belmont Stakes, Napravnik became the first female jockey to ride in all three Triple Crown races, booting Todd Pletcher-trained filly Unlimited Budget to sixth.

Without question, Napravnik is the most talented female rider to come along since Julie Krone, a Hall of Famer who now lives in Carlsbad. It’s not even close, although there have been and are some terrific female jockeys, especially on the East Coast and in the Midwest, where most of the female jockeys ride.

For many of them, the issue of why there aren’t more women in horse racing is a sensitive one.

“I have no idea,” Napravnik said, bristling at the question after a race at Santa Anita this spring. “I think that is the worst question … ever. Julie Krone and all the female riders who were successful before me were able to get themselves established at a very high level.

“And I’ve ridden with some great female riders: Rosemary Homeister, Emma-Jayne Wilson, Chantal Sutherland (who is making a comeback at Del Mar and will be a focus of the track’s publicity campaign for the second straight year) and Inez Karlsson. It’s just a really unfair question.”

Krone laughed loudly when she heard what Napravnik had said when asked the question. Krone thinks the world of Napravnik and cheers for her enthusiastically, even if it means cheering for a woman rider who could break most of the records that helped Krone become the first female jockey to make horse racing’s Hall of Fame. Napravnik finished eighth in earnings last year with nearly $12.5 million, the highest ever for a female rider. This year, she’s fifth with more than $7 million, with a lot more racing to go.

Krone, who experienced much sexism when she was coming up, said that part of the game has improved. But there are still other reasons why women have trouble making it.

The biggest, she said, is strength. A jockey must have a strong upper body and legs to properly handle a horse. While jockeys are famous for being short and light, men are generally stronger than similarly sized women.

“The first time I ever rode next to Angel Cordero, I heard his racing silks make this noise as he snapped his arms,” Krone said, making the motion of a rider. “It makes a different sound. You ride next to that and you say, ‘Wow, I have a ways to go as an athlete.’ ”

Longtime female jockey Zoe Cadman agrees with Krone’s evaluation. She said skill is the main reason why more women aren’t riding horses in the biggest races.

“It’s not the fact that the women don’t get a shot,” said Cadman, who now is an analyst and host for HRTV (a horse sports network). “That’s not the point. It’s just that the guys are better. You get good jockeys and bad jockeys. Some of them happen to be guys. Some of them happen to be girls. It’s not really a question of gender. I hate to be this blunt, but women just aren’t that good.”

Of course, many women point out that they have other factors to deal with during their peak professional years — like having babies. That was the case for Kayla Stra and Tammi Piermarini, both promising jockeys who took time off to start families. With jockeys sometimes riding in hundreds of races in a year, taking a break costs them not only time, but also business when trainers look elsewhere for a rider.

Stra recently found herself embroiled in a controversy when she took her baby boy to the women’s jockey room at Betfair Hollywood Park. Steward Scott Chaney said it was a misunderstanding, but Stra felt she was being discriminated against when the stewards told her she needed to breast-feed or change her baby’s diapers in a room separate from the women’s jockey room. That caused California Horse Racing Board Chairman David Israel to issue a directive to the Hollywood Park stewards that Stra was to be allowed in the women’s jockey room with her baby and also permitted to perform her regular motherly duties.

Del Mar officials have told Stra that she is welcome to have her baby in the women’s jockey room at the meet that starts Wednesday.

Stra paid her dues racing in Australia, where she said she won nearly 500 races, before moving her business to Southern California in 2007.

“I’m just trying to get this incident behind me,” Stra said. “I wish I didn’t have to go through all that. It’s all about riding, really.”

For the most part — and women find this out as well as men — horse racing is the ultimate “gotcha” game. When a rider is out with an injury or anything else, it opens a saddle for another rider. The game waits for no one.

“A woman has to be really tough to make it in racing because those guys in the jock’s room are so tough,” said Chris Aplin, a former exercise rider who wanted to be a jockey at one time. “They’ll do anything to beat you.”

Those who knew Krone when she was at the top of her game agreed that she was as tough as the men were on younger jockeys, including female riders. She tested them the way men tested her.

Krone said that’s one of the reasons why she and Napravnik have succeeded where so many other women have failed. She calls herself and Napravnik the “archetype for women jockeys.” They grew up on horses. Krone was standing on a pony when she was a toddler. Napravnik raced ponies as a kid. They realized early what it took to handle a horse and later to compete against boys and then men.

Krone and Napravnik share two innate qualities — gritty determination to be better horsewomen and an insatiable hunger to be great and to win at any cost.

“There is a presence, a conscious unconsciousness that an athlete can produce something without explanation,” Krone said. “Are they Mozart? Are they given some super strength or ability? Where does the natural talent come in and where does the oppression of the gender come in to such a severity where it inhibits the girl to do the crossover, where she can make the trade-off to know that, ‘I’m going to ride this thoroughbred with reckless abandon. I’m going to ride this thoroughbred and trade a body part or risk getting mentally deranged in order to share this relationship with this horse.’ Where does it come from? It can start in the womb, for all we know.

“But I know one thing. It takes both genders to ride a racehorse, the feminine for soft and subtle, and the masculine for strong and effective. So, if you think about it, girls are going to be naturally gifted with thoroughbreds.”

There is some evidence that the horse racing world is starting to recognize that. Wilson, a Canadian rider, is a rising star and the first woman to win the Queen’s Plate, Canada’s oldest race and the country’s version of the Kentucky Derby.

“There are a larger number of girls getting the opportunity to ride and showcase their abilities, and within that percentage you get those upper-echelon jocks that get a chance to showcase themselves in the big races,” she said.

Napravnik is savvy enough to know that her success will influence the sport. If she keeps winning consistently and placing well in the biggest races, not only will more girls be drawn to the sport, but more owners will have confidence that women can ride on the biggest stage.

“We just don’t have the numbers,” said Diane Crump, who was 19 when she became the first woman to ride in a pari-mutuel thoroughbred race, on Feb. 7, 1969. “But Rosie has a chance to really do something special. I love the way she’s low-key. I love her attitude and her personality. She reminds me of me when I was riding. She works her butt off.

“As she wins more and wins a big race like a Triple Crown race, she’ll realize her value to the sport. The sport needs her, and she’ll realize she can make a difference.”